The preferred embodiments relate to power electronic systems and methods, such as those driven with power field effect transistors (FETs).
Some electronically-driven power devices incur high transient conditions. For example, in automotive body module applications, such as energizing an incandescent bulb coil at cold temperatures, very high peak in-rush current may be required to initially drive the coil, such as current demands in the range of approximately 90 A to 100 A. Thus, circuit demands, such as at cold start-up, tend toward requiring high current flow to meet the device (or customer) demands. Typically, a high-side power FET is used as switch to allow this much current to flow. The need for, and provision of, high current levels, however, raises other design complexities. Particularly, in high current applications, it is recognized that in some circuit conditions, current level can exceed even the anticipated high supply provided. For example, if a true short-circuit develops in the load, then very large amounts of current may flow, while that current is sourced immediately to ground via the short. As another example, where the load is inductive, as can be the case for an incandescent bulb, then there is a sudden negative voltage spike across the inductive load when its supply voltage is suddenly reduced or removed—a condition known as flyback.
Given the preceding considerations, some effort exists in the prior art to include some type of control on the FET gate, so that current can be limited in some measure so that it does no reach a level that could damage the FET, the load, or other related circuitry. One such approach provides an analog control, which attempts to regulate the FET sourced current to not exceed a particular level. This approach, however, is not always effective as the analog nature may permit some deviation in the amount of sourced current. Another approach disables current flow in response to instantaneous current or power exceeding a set threshold. This approach, however, dictates a high current threshold, and such a threshold, therefore can lead to very high voltage across the FET in instances other than the in-rush event, such as the true short circuit or flyback. These high voltage events, therefore, can stress, damage, or otherwise violate the safe operating area (SOA) boundary violations of the FET, for example, during switch turn-on, switch turn-off, and other events. Moreover, the instantaneous nature of such a circuit causes a shutdown of current flow when the monitored threshold is exceeded, followed typically by a delay and re-try, that is, where power is restored following the threshold-detection. However, if the current demands of the circuit rise quickly yet for a short time, the protective circuit may immediately respond by disabling current flow, then re-try only to repeat the disablement, with the process causing repeated failures in sourcing current that is otherwise needed for normal operation of the application. Moreover, repeated re-tries, that is, repeated application of current into a short circuit, may cause large thermal accumulation on the power circuit, also tending toward circuit damage.
Given the preceding, while the prior art approaches may be acceptable in certain implementations, some applications may have requirements that are not sufficiently addressed by the prior art. Alternatively, such approaches may be deemed unacceptable to an electronics customer seeking to implement an application. Thus, the present inventors seek to improve upon the prior art, as further detailed below.